Microstructural and mechanical properties of gravity-die-cast A356 ...

A R C HIVE S of F OUND R Y E NGINE E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310) Volume 11 Issue 4/2011 77 – 82

16/4

Microstructural and mechanical properties of gravity-die-cast A356 alloy inoculated with yttrium and Al-Ti-B grain refiner simultaneously Y.P. Lim*, J.B. Ooi, X. Wang Department of Mechanical Engineering, Universiti Tunku Abdul Rahman, Kuala Lumpur, 53300 Malaysia *Corresponding author. E-mail address: [email protected] Received 25.05.2011; accepted in revised form 27.07.2011

Abstract In the present work, the effect of inoculating yttrium and Al-5Ti-1B simultaneously on A356 aluminum alloy has been studied. Gravity die casting process is used to cast the ASTM tensile test specimens for analysis. In each experiment, the Ti and B contents were maintained constantly at 0.1 and 0.02 wt% respectively. The addition of yttrium was manipulated at the amount of 0, 0.1, 0.2, 0.3, 0.4 and 0.5 wt%. M icrostructural characterization of the as-cast A356 alloy was investigated by means of optical microscope and its phases are detected by XRD. The mechanical properties tested are tensile strength and hardness. The inoculation of yttrium was found to enhance the grain refinement effect of Al-5Ti-1B grain refiner and improve the mechanical properties. The optimal weight percentage of yttrium was found to be 0.3. The grain refining efficiency of combining yttrium and Al-5Ti-1B on A356 aluminum alloy was mainly attributed to the heterogeneous nucleation of TiB2 and TiAl3 particles which were dispersed more evenly in the presence of yttrium and the grain growth restriction effected by the accumulation of Al-Y compound at grain boundaries. Keywords: A356, Al5Ti1B, Yttrium, Grain refinement, Gravity dies casting, XRD

1. Introduction Grain refinement plays an important role to determine the quality and integrity of aluminium alloys. It is always desirable to achieve grain structure which is fine and equiaxed in the as-cast aluminium alloys products produced by various casting processes. A coarse grained structure may result in a variety of surface defects and higher tendency of hot cracking in the castings produced. Twinned columnar grains have been known to reduce fabricability, yield strength and tensile elongation and stress at fracture. On the other hand, grain refinement will offer a number of advantages in foundry operations such as smoother feeding of

A RCHIV ES o f F O UNDRY ENG INEERI NG

molten materials, reduced chemical segregation, minimization of porosity and elimination of hot tearing [1]. The refinement of grain size can be achieved by several methods like inoculation with grain refining agents, solid solution heat treatment and artificial aging, plastic deformation using processes such as hydrostatic extrusion etc [2-3]. Grain refinement resulting from solidification assisted by proper inoculation of heterogeneous particles into the melt is still the most ubiquitous technique used in foundry industry to improve the mechanical properties of aluminium castings.

V o l u me 1 1 , Is s u e 4 / 2 0 1 1 , 7 7 - 8 2

77

Aluminium alloys constitute a significant proportion of lightweight metals used in various industries for applications ranging from automotive components to aerospace parts etc [4]. There is always a constant need to improve the microstructure and its inherent mechanical properties of aluminium alloys castings due to the increasing awareness of reducing green house emissions by using lightweight materials in automotive industries. Aluminium alloys are known to have excellent strength to weight ratio compared with other conventional metals like steels. A356 is one of the most widely used aluminium alloys in many industrial applications because of its excellent castability, corrosion resistance and good mechanical properties. It has lower production cost, fast machining rate and good recyclability. Typical commercial grain refiners use to refine A356 aluminium castings are Al-Ti-B and Al-Ti-C master alloys. The efficiency of these grain refiners can be easily undermined by alloying elements like Zr and V [5]. In recent years, yttrium has been regarded as a promising element in superalloys for its ability to improve creep property and oxidation resistance of cast stainless steel [6-7]. However, very little work was done to investigate the effect of yttrium on the grain refining efficiency of Ti-B based grain refiner in A356 casting. Therefore, the research presented by this paper intends to study the effect of rare earth yttrium on the grain refinement efficiency of Al-T-B master alloy by using gravity die casting as the casting process.

2. Experimental Procedures In this study, the commercial A356 aluminium alloy was used as the base metal in all castings. The liquidus and the solidus temperatures of the alloy were found to be 615 oC and 538.5oC respectively according to manufacturer’s data. The grain refiners used are Al5Ti1B master alloy supplied by KBM AFFILIPS. The rare earth used is Yttrium of 99.9% purity. The manufacturer’s data of the compositions of the A356 alloy, Al5Ti1B master alloy are given in Table 1. Table 1. Compositions of A356, Al5Ti1B and Tical315 Elements, Si Fe Mn B wt% A356 7.22 0.15 0.01 Al5Ti1B 0.1 0.16 1.0 TiCal315 0.05 0.16 0.01 0.0005 Ti Ni Zn Sr Mg 0.13 0.016 0.04 0.01 0.45 5.0 3.1 -

C 0.15 Al Bal. Bal. Bal.

Fig. 1. Gravity die casting mould The surface of the mold was coated with a layer of mold release agent in order to facilitate casting knock-out after pouring and solidification. The A356 aluminium alloy was put into the crucible of a Nabertherm electrical furnace and melted up to 750 ± 5 oC. After complete melting, calculated fixed quantity of Al5Ti1B which constitutes the weight percentages of 0.1 Ti and 0.02 B were added into the melt and stirred for 30 seconds before the addition of yttrium. The quantity of yttrium was added as 0.1, 0.2, 0.3, 0.4 and 0.5 wt% in separate experiment. The molten alloy was then directly poured into the gravity die casting mold. The castings are purposely designed for ultimate tensile strength test. They were subjected to fettling and cleaning and subsequently machined to a diameter of 20mm at the gripping ends. The as-cast samples and machined samples are shown in Figure. 2. The tensile test was done on INSTRON 5582 with a tensile rate of 2mm/min. The central part of the tensile specimen was cut to a thickness of 10 mm and subjected to fine 80 grit -size grinding on both sides to smoothen the coarse surfaces for hardness test. The hardness test was done on Indentec Universal Hardness Tester. The scale of all tests were set to be HRA 60 kgf. A sample of size 5mm x 5mm was cut from the transverse plane at the central part of each tensile specimen and mounted in resin to prepare for grinding, rough polishing and finally fine polishing to the fineness of 0.3 micron. The polishing agent was buehler alpha alumina particles of 0.3 micron. The samples were chemically treated with etchant consisting of 200 ml distilled water and 5 ml HF9. M icrostructural studies were conducted by using an optical microscope with a maximum magnification power of 2000X. Similarly polished samples of 0.3 mm thickness were used for XRD examination.

The gravity die casting mold used is designed according to JIS H5202 standard which contains two cavities of cylindrical shape tensile test piece of gage length 50 mm and diameter 14mm. The internal configuration of the mold is shown in Figure 1.

Fig. 2. Casting samples

78

A RCHIV ES o f F O UNDRY ENG INEERING

V o l u me 1 1 , Is s u e 4 / 2 0 1 1 , 7 7 - 8 2

The main results obtained from this study are the mechanical properties of ultimate tensile strength, hardness and elongation (strain at fracture). The original A356 data is used as benchmark to determine the performance of yttrium on Al5Ti1B grain refinement in A356 gravity dies casting. The microstructures taken by optical microscope will be compared to analyze the effect of yttrium on grain size. XRD analysis results is used to find out is there any special phase formed in the casting after adding yttrium as inoculants.

3.1. Hardness The hardness of original A356 is 19.84. The 0.1wt% Ti and 0.02wt% B increases the hardness to 21.62. Keeping the contents of Ti and B constant, the addition of yttrium from 0.1 to 0.3wt% also shows improvement in hardness with the values of 21.10, 22.16 and 23.88 respectively. However, continuous addition of 0.4 and 0.5wt% does not further improve hardness but reduces it to 18.02 and 17.08 respectively. The results show that the best hardness value is obtained by inoculating 0.3wt% of yttrium in combination with 0.1wt% Ti and 0.02wt% B into the A356 casting. The graph in Figure 3 shows the data of hardness vs. yttrium wt%. 0wt% refers to the A356 casting without any grain refiners. This applies to other graphs in the following sections.

Hardness of yttrium grain-refined A356

without grain refinement but lower than that added with 0.3wt% yttrium. This indicates that yttrium is effective to improve the tensile strength of A356 casting by a maximum of 40%. Based on the microstructures shown in section 3.4, the addition of yttrium results in finer dendritic structure and more fibrous eutectic phase, this contributes to strengthening effect on the casting. Coarse and elongated silicon particles in eutectic phase of the unmodified A356 tends to fracture at lower tensile force while the refined and more fibrous silicon particles in yttrium-modified A356 are more resistant to dislocation under tension. The specimen which is solely grain-refined by Al5Ti1B also develops a better tensile strength as a result of finer dendritic structure due to the heterogeneous nucleation promoted by Al3Ti and TiB2 particles exist in the melt during solidification. When >0.3wt% yttrium is added, the tensile strength drops. This could be due to the intermediate compound containing yttrium aggregates and grows, cuts up the α-Al matrix and weakens the obstruction to the boundary movement [8].

UTS of yttrium grain-refined A356

UTS, MPa

3. Results and discussions

200.0 180.0 160.0 140.0 120.0 100.0 80.0 60.0 40.0 20.0 0.0 0

25.0

0.1

0.2

0.3

0.4

0.5

HRA (60 kgf)

24.0

Yttrium

23.0

Addition level, %wt

22.0 21.0

Fig. 4. Ultimate tensile strength

20.0 19.0 18.0

3.3. Elongation

17.0 16.0 15.0 0

0.1

0.2

0.3

Yttrium

0.4

0.5

Addition level, wt%

Fig. 3. Hardness

3.2. Tensile Strength Two tensile test samples for each type of alloy were subjected to test and the averaged values are taken to plot the ultimate tensile strength chart as shown in Figure 4. It can be seen that 0.1wt% yttrium does not improve the tensile strength much, it has a tensile strength of 122.65 M Pa while the original A356 has 123.49 M Pa of tensile strength. The tensile strength increases when 0.2 and 0.3wt% of yttrium is added, it improves to 148.29 and 173.40 M Pa respectively. However, further addition of yttrium does not improve tensile strength, instead, the tensile strength drops to 144.52 and 127.92 M Pa for 0.4 and 0.5wt% of yttrium. The A356 containing 0.1wt% Ti and 0.02wt% B has a tensile strength of 153.18 M Pa, it is higher than the original A356


Ductility of a metal can be measured by its elongation or strain at specific point in the stress –strain curve. In this study, strain at fracture is taken into consideration to analyze the effect of yttrium on the ductility of A356. The original unrefined A356 has a tensile strain of 0.064. When it is added with TiB grain refiner to contain 0.1wt% Ti and 0.02wt% B, the ductility does not improve significantly; its tensile strain is 0.069. When yttrium is added from 0.1 to 0.5wt%, the tensile strain improves slowly to the maximum value of 0.076 at 0.3wt% yttrium. Similarly to harness and tensile strength, further addition of yttrium exceeding 0.3wt% does not improve ductility continuously. The strain behaviour of yttrium-modified A356 is shown in Figure 5.

V o l u me 1 1 , Is s u e 4 / 2 0 1 1 , 7 7 - 8 2

79

Elongation of grain-refined A356 Strain at UTS, %

0.090 0.080 0.070 0.060 0.050 0.040 0.030 0.020 0.010 0.000 0

0.1

0.2

0.3

0.4

0.5

Addition level, %wt

Yttrium

Fig. 5. Elongation at fracture

3.4. Microstructural Analysis The microstructures of the A356 and its Y-Ti-B-grain refined specimens are shown in Figure 6 (a)-(g) (all scale lengths are 0.133 mm). The original A356 gravity die casting has coarse α-Al dendritic microstructure with very fine and rod-like eutectic phase. The addition of 0.1 wt% Ti and 0.02wt% B into the melt is observed to have refined the dendritic structure. It is still a dendritic structure but characterized with smaller secondary dendrite arm spacing. The addition of yttrium from 0.1 to 0.5wt% into the melt is seen to have refined the microstructure potently. Yttrium is found to be able to reduce the secondary dendrite arm spacing from the unrefined A356 coarse SDAS of 0.037 mm to the finest SDAS of 0.01 mm in 0.3, 0.4 and 0.5wt% Y-refined casting. Smaller SDAS is known to strengthen the casting and produce better mechanical properties of tensile strength and elongation. Increasing amount of yttrium is observed to decrease the eutectic silicon phase and the grain boundary becomes narrow and continuous. In this investigation, yttrium is found unable to transform the α-Al phase from dendritic structure into globular structure. Previous study showed that such a globular transformation in A356 can only be achieved by T6 heat treatment [9]. Yttrium is found to be able to modify the silicon eutectic phase into a more fibrous form as compared with that of unrefined A356. Adding yttrium into A356 will produce the high-melting point Al-Y compounds at the solid/liquid interface and lead to the formation of solute undercooling layer, suppression of the growth of α-Al grains and subsequently grain refine the dendritic structure of α-Al matrix [10].

(a)A356

(b) 0.1wt% Ti + 0.02wt%B

(c) 0.1wt% Y

80


V o l u me 1 1 , Is s u e 4 / 2 0 1 1 , 7 7 - 8 2

(d) 0.2wt% Y

(g) 0.5wt% Y Fig.6 (a)-(g). M icrostructures of test specimens

3.5. XRD Analysis

(e) 0.3wt% Y

A typical XRD diffractogram for 0.3wt% yttrium inoculated A356 is shown in Figure 7. The search function of the XRD software does not detect any significant yttrium-aluminium based compounds. Based on the searched results for all samples, the following compounds are found: Al9Si, Al0.86Zn0.14, Al0.95M g0.05, Al0.86Zn0.14 and Ti. It is the limitation of the XRD machine used for not being able to detect the low contents of yttrium and its associated compound. However, when comparing with the XRD diffractogram of other researchers [10], the spectrum looks similar and it is expected to contain the phases of α-Al, TiAl3, AlY3, AlY2, TiC and TiB2. It has been reported that the intermediate compound of AlY that aggregates along grain boundaries and hence enhances grain boundaries to resist slipping or dislocation [11]. This in turn will improve the tensile strength of the casting.

Fig. 7. XRD diffractogram of 0.3wt% Y (f) 0.4wt% Y


V o l u me 1 1 , Is s u e 4 / 2 0 1 1 , 7 7 - 8 2

81

4. Conclusions The combined effect of grain refiners Al5Ti1B (fixed at 0.1wt% Ti and 0.02wt% B) and yttrium on the mechanical properties of A356 gravity die castings has been studied. Based on the mechanical testing and metallographic examination conducted for the specimens, the following conclusions can be drawn: 1. 0.3wt% yttrium renders the highest hardness value of 23.88 HRA (60kgf), a 20% improvement compared to A356. 2. Grain refinement with 0.3wt% addition of yttrium shows the greatest improvement in tensile strength too by 40%. This shows that yttrium can significantly improve tensile strength of A356. 3. 0.3wt% yttrium also yields the best ductility. However the improvement is not significant, it improves only by 0.012 mm/mm. 4. M icrostructural analysis shows that yttrium is able to refine grain size by reducing the SDAS and produces more fibrous eutectic silicon phase. However, the dendritic structure of αAl is still unchanged. 5. Addition level of >0.3wt% yttrium will not further improve the mechanical properties of A356.

References [1] Yücel Birol, “Production of Al-Ti-B master alloys from Ti sponge and KBF 4”, Journal of Alloys and Compounds 440 (2007), pp. 108-112.

82

[2] T.R. Ramachandran, P.K. Sharma, K. Balasubramaniam, “Grain Refinement of Light Alloys”, 68th WFC – World Foundry Congress, Feb 2008, pp. 189-193. [3] M oustafa M A, Samuel FH, Doty HW, “Effect of solution heat treatment and additives on the microstructure of Al-Si automotive alloys”, Journal of M aterials Science 2003, Vol 38, pp. 4507-4022. [4] M iller WS, Zhuang L, Bottema J, et al, “Recent development in aluminum alloys for the automotive industry”, M aterials Science and Engineering A: 2000; Vol 280(1): pp. 37-49. [5] Yücel Birol, “Grain refining efficiency of Al-Ti-C alloys”, Journal of Alloys and Compounds 422 (2006), pp. 128-131. [6] W.O. Ngalaa, H.J. M aier, "Creep–fatigue interaction of the ODS superalloy: PM 1000 ", M aterials Science and Engineering A, Vol 510-511, 15 June 2009, Pages 429-433. [7] P.J. Zhou, J.J. Yu, X.F. Sun, et al, “Role of yttrium in the microstructure and mechanical properties of a boronmodified nickel-based superalloy”, Scripta M aterialia 57 (2007), pp. 643-646. [8] Li Hui-zhong, Liang Xiao-peng, Li Fang-fang, et al, “Effect of Y content on microstructure and mechanical properties of 2519 aluminum alloy”, Transaction of Nonferrous M etals Society of China, Vol 17, 2007, pp. 1194-1198. [9] Heat Treating, M etals Handbook, ninth edition, ASM International, M etals Park, OH, volume 4, 1981. [10] Xu Chunxiang, Lu Binfeng, Lü Zhengling, et al, Journal of Rare Earths, “Grain refinement of AZ31 magnesium alloy by Al-Ti-C-Y master alloy”, Vol. 26, No. 4, Aug 2008, p. 604. [11] Chen Yu-yong, Si Yu-feng, Kong Fan-tao, et al, “Effects of yttrium on microstructures and properties of Ti-17Al-27Nb alloy”, Transaction of Nonferrous M etals Society of China, Vol 16, 2006, pp. 316-320.


V o l u me 1 1 , Is s u e 4 / 2 0 1 1 , 7 7 - 8 2